An ion implantation process is a process used in a device manufacturing to implant one or more desired species into a target to change at least one of its electrical, optical, chemical, and mechanical property. Depending on the device, the target may be a substrate such as a wafer, or a film or other material formed thereon. In solar cell manufacturing, the target may be silicon or another semiconducting substrate, and ion implantation process is used to alter the optical and/or electrical property of the substrate.
One type of the solar cell that can be manufactured using ion implantation process is an interdigitated back contact (IBC) solar cell. Referring to FIG. 1, there is shown a cross-sectional view of a conventional IBC solar cell 100. The IBC solar cell 100 comprises an n-type base 102. On the front side of the IBC solar cell 100, there may be an n+ front surface field (FSF) 104, a passivating layer 106, and an anti-reflective coating (ARC) 108. Although various dielectric materials may be used as the passivating layer 106, conventional IBC solar cell 100 may include SiO2 as the pasivating layer 106.
On the back side of the IBC solar cell 100, one or more p+emitter 112 and one or more n+ back surface field 114 disposed alternately to form one or more p-n junctions therebetween. Behind the p+ emitter 112 and n+ BSF 114, there may be a passivating layer 122 with a plurality of contact holes 124. A p-type contact finger 126 and an n-type contact finger 128 may be formed behind the passivating layer 122. Each of the p-type contact finger 126 and the n-type contact finger 128 may be contact with the p+ emitter 112 and n+ BSF 114, respectively, via the contact holes.
In operation, the IBC solar cell 100 is positioned such that the front side of the IBC solar cell 100 is exposed to sunlight 132. The sunlight 132 enters the solar cell 100 through the n+ doped region, also known as the front surface field (FSF) 102. Photons in the sunlight with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material's valence band to the conduction band creating electron-hole pairs. In order to generate a photocurrent that can drive an external load, these electron-hole pairs need to be separated. This is done through the built-in electric field at the p-n junction. Thus, any electron-hole pairs that are generated in the depletion region of the p-n junction get separated, as are any other minority carriers that diffuse to the depletion region of the device. Since a majority of the incident photons are absorbed in near surface regions of the device, the minority carriers generated in the emitter need to diffuse to the depletion region and get swept across to the other side. As a result of the charge separation caused by the presence of this p-n junction, the extra carriers (electrons and holes) generated by the photons can then be used to drive an external load to complete the circuit.
In a conventional IBC cell 100 manufacturing process, each of the p+ emitter 112 and the n+ BSF 114, which have complex geometries and which are alternately disposed, may be generated using the ion implantation process. To achieve their geometries, two patterned or selective ion implantation process are performed. In the first patterned ion implantation process, a first mask (not shown) is disposed on the base 102, which is initially doped with n-type dopants. The first mask contains an aperture that corresponds to the geometry of the p+ emitter 112. After the mask is disposed, the base 102 is implanted with p-type dopants, resulting in p+ emitter 112, and the pattern of the mask is transferred onto the base 102. Thereafter, a second mask (not shown) having an aperture corresponding to the geometry of the n+ BSF 114 is disposed. The base 102 is then implanted with n-type dopants, and the second patterned or selective ion implantation is performed.
The process noted above requires many steps, each of which may be expensive. Any reduction in the number of the steps included in the manufacturing of the IBC solar cells would have a positive impact on lowering the cost of the solar cells and the implementation of solar cells. This will enable the wider availability of this clean energy technology. As such, new techniques are needed.